The quality of digitally processed images of documents depends on the nature of the original document, the digitization process (including an image processing algorithm), noise introduced between digitization and display, and the quality of the display device. One of the most effective methods for assuring good image quality is to use a high sampling density during digitization and a high resolution display device. However, the costs for data storage, data processing, data transmission, and display hardware can be prohibitive. Accordingly, efforts have been made to produce satisfactory images at lower cost.
A considerable amount of work has been devoted to determining the minimum requirements for the display of bi-level (1 bit/pixel) digitized images of text. For example, it has been found that a minimum pixel frequency of 110 pixels per inch (ppi hereinafter) was required for 97.5% legibility of standard typewritten and printed materials. A pixel frequency of 125 ppi was found to be necessary for 99% correct recognition. (Arps et al., "Character Legibility Versus Resolution in Image Processing of Printed Matter", IEEE Transactions on Man-Machine Systems, volume 10, page 66-71 (1969)).
Both the legibility and the subjective appearance of digitized images of text can be improved by increasing input and output resolution. Creating levels of grey may have the same effect under certain circumstances. Grey levels may be created from bi-level (black and white) text documents by prefiltering during the initial digitization process or by post filtering.
It has been found that digitally processed images of handwritten and machine-printed characters are more legible when the number of grey levels was increased from two (1 bit/pixel) to eight (3 bits/pixel). The effect was most pronounced when the sampling density was relatively low (i.e., 80 samples/inch). Under these conditions, the error rate for character recognition was decreased from 26% to 19%. (Lee et al., "Digital Image Quality as a Function of Sampling Density and Grey Levels", Occupational Health and Safety, Proceedings of the Twelfth Annual Meeting of the Human Factors Association of Canada (pages 9.1-9.5), 1979.) Similarly, it has been shown that the subjective quality of digitized images of text could be improved by using four or more grey levels (i.e., at least 2 bits/pixel). (Warnock, "The Display of Characters Using Grey Level Sample Arrays" Computer Graphics, Siggraph '80 Conference Proceedings (pages 302-307), (1980)).
There is a need for a method and apparatus for processing and displaying digital images of text and graphics for imaging applications involving electronic displays that have moderate spatial resolution and grey scale capability so that business documents can be displayed at lower cost than with high resolution bi-level black and white monitors that are typically used.